I. Field of the Invention
In general, the invention relates to the field of obtaining purified materials from recycling, and more specifically, obtaining purified caprolactam from recycled nylon, as found in nylon carpeting.
The recycling nylon has been the focus of many years of research because of the simplicity of the concept and the attractiveness of potential economic benefits. The annual energy impact of this technology in the year 2010 is estimated to be approximately 23 trillion BTU for the expected production of 400 million pounds of .epsilon.-caprolactam from waste carpet that is currently accumulating in landfills. This estimated energy consumption is based on the current production method for nylon-6 fiber from fossil resources.
In the recycling scheme, one processing step replaces nine processing steps in the production of .epsilon.-caprolactam from crude oil. Thus, the 400 million pounds of .epsilon.-caprolactam anticipated to be produced in 2010 can be produced from about 800 million pounds post-consumer carpet waste, while the non-nylon components of the carpet could be used to supply process energy.
II. The Prior Art
In the depolymerization of nylon-6 of waste carpet through pyrolysis to obtain .epsilon.-caprolactam monomer for repolymerization into nylon, a significant obstacle is the formation of the undesirable by-product 1,11-diamino-6-undecanone in its amine form, referred to hereinafter as "imine".
In other words the caprolactam resulting from the depolymerization requires further purification before it can be effectively and efficiently repolymerized.
From Rudolph Puffr and Vladimir Kuba'nek, Lactam-Based Polyamides, Volume I and II; CRC Press, Inc.: Boca Raton, 1991, it was known that the main products of polyamide 6 burning (1000.degree. C.) are C.sub.2, caprolactam, and aromatic hydrocarbon; however, the possibility of the formation of hydrogen cyanide is acute during high temperature polyamide oxidation in a closed system when ammonia is formed from the --NH.sub.2 end groups. Namely, when polyamide 6,11, 66 or 610 was oxided at 500.degree. C. in the presence of NH.sub.3 (0.5-5 g/100 g of polymer) considerable amounts of HCN were generated (2-4 g/100 g of polymer). Therefore, reliance on this process to provide caprolactam is fraught with the prospect of poisonous products such as HCN.
A process of using fast pyrolysis in a carrier gas to convert a polyamide containing a plastic waste feed stream comprising nylon 6 in a manner such that pyrolysis of the polyamide produces the high monomeric constituent caprolactam is disclosed in U.S. Pat. No. 5,216,149. However, the pyrolyzate of this process requires further purification to extract pure-crystalline caprolactam as the removal of impurities is crucial to the reusability of the caprolactam for production of nylon-6.
U.S. Pat. No. 5,280,105 discloses a method for separating nylon 6 polymer from a mixture thereof with nylon 6,6 polymer, comprising treating the mixture with an aqueous solution of an aliphatic carboxylic acid at a concentration and temperature sufficient at atmospheric pressure to dissolve nylon 6 polymer, while leaving nylon 6,6 and separating the solution of nylon 6 from the nylon 6,6 polymer. In other words, the separation of nylon 6 from nylon 6,6 is based on the differences in the solubilies of these nylon materials.
A process for forming a thermoplastic composition from carpet wherein a carpet sample is melt-blended without separating the carpet into its component parts is disclosed in U.S. Pat. No. 5,294,384. The carpet sample is preferrably a conventional sample of carpet comprising nylon tufts formed from nylon 6 or nylon 6,6 or blends or copolymers thereof, and at least one polyolefin backing material and a styrene-butadiene rubber (SBR) adhesive originally applied as a latex.
However, there is a need extant in the art of utilizing caprolactam from recycled nylon for repolymerization into nylon 6, to devise an efficient process for the selective removal of 1,11-diamino-6-undecanone from .epsilon.-caprolactam generated through pyrolyis of nylon carpet. The process for selective removal of the 1,11 diamino-6-undecanone to effect purification of the final caprolactam containing pyrolyzate would constitute a critical step in developing this technology for commericalization for repolymerization of the caprolactam to nylon.
Recycling of nylon-6 containing carpet waste through pyrolysis technology leads to excellent yields of .epsilon.-caprolactam, hereinafter referred to as caprolactam; however, purification of the final caprolactam containing pyrolyzate has proven to be very critical in developing this technology for commercialization.